Add for statement and range function; New register_box_fn for functions returning Box<dyn Any> (which should not be boxed again); Register standard utility functions in library (e.g. print, string concat).

scripts/for1.rhai

@@ -0,0 +1,15 @@
+let arr = [1,2,3,4]
+for a in arr {
+    for b in [10,20] {
+        print(a)
+        print(b)
+    }
+    if a == 3 {
+        break;
+    }
+}
+//print(a)
+
+for i in range(0,5) {
+    print(i)
+}